A significant amount of research has been conducted on processes for the orientation of polyethylene molecules, for purposes of increasing the strength of the products manufactured therefrom. It is known that the covalent carbon-carbon bond is one of the strongest of all chemical bonds. Theoretical elastic modulus and strength along the chain of the polyethylene molecule reaches values as high as 300 and 10 GPa respectively. In practice, however, it is very difficult to manufacture a product having even 10% of the theoretical strength and modulus, at least in an economically attractive, commercial process.
Orientation of polyethylene molecules can be pictured as an alignment of the molecules, so as to be disposed with the axes of their main chains generally parallel to one another, in the mass of the polymer. Such orientation causes quite dramatic increases in the mechanical strength properties of the polymer as is well known.
Polyethylene products in general are subject to "creep," i.e. they gradually elongate under stress over time due to the chain extension and slippage of the molecules. This is particularly troublesome in applications such as racquet strings and binding strapping, where a slackening of the tension imparted on application can render them useless, and in applications where they are to be bonded to other plastics of different physical characteristics.
Though increase of orientation results in a great improvement of creep resistance, highly oriented polyethylene products still show sufficient creep to cause major problem in load bearing applications. Previous studies have indicated that crosslinking improved the creep resistance and the thermal resistance of oriented and unoriented polyethylene.